1. Field of the Invention
The present invention relates to an antenna for a base station of mobile communication system, and in particular, to an antenna apparatus for a base station suitable for narrow service areas, or to an antenna apparatus for a relay station in dead zones.
2. Description of the Related Art
Antennas for the base stations of mobile communication systems are classified into base station antennas for the purpose of covering large-scale regions and base station antennas for the purpose of covering relatively limited, small-scale regions. The antennas for covering the small-scale area are used for covering so-called radio “dead zones” which are building shadow regions where radio waves are blocked by tall buildings. In metropolitan areas, since the radio dead zones are dispersed at relatively short distances, a concept known as a multi-hopping method has been considered, in which relay stations equipped with antennas covering small-scale regions are disposed at every radio dead zone and then the relay stations relay between one another in order to eliminate the dead zones.
Monopole antennas and dipole antennas are being used as base station antennas suitable for the radio dead zones or the narrow service areas. US Patent Publication No. 2005/0156804 A1 discloses designs for a radiation element of a monopole antenna with a finite ground plate. When such a monopole antenna is used at, for example, base stations for narrow service areas or relay stations using the multi-hopping method, the monopole antenna needs to be set up as a stand-alone antenna if the walls of the buildings cannot be used for the installation of the antenna.
FIGS. 8A and 8B are views illustrating a conventional monopole antenna being installed as a stand-alone type antenna, and characteristics thereof. In FIG. 8A, the monopole antenna 92 is arranged at a top end of an antenna support of the stand-alone type antenna standing on a ground plane 90, and is supplied with RF power through a feeding line 93 from a base station or relay station (not shown). Since a finite ground plate is arranged at the ceiling, a radiation pattern in the X-Z plane is characterized by a main lobe directed downward from a horizontal direction which is suitable for radiating radio waves toward valleys between buildings. FIG. 8B shows radiation pattern characteristics illustrating a radiation pattern in the X-Y plane of the installed monopole antenna 92. The monopole antenna 92 shows an omni-directional (all directional) radiation pattern in the shape of a circle, but the support 91 interferes with the radio waves, whereby attenuation of radio waves arises in −X direction, and the circular shape is distorted. This attenuation causes the deterioration of communication quality or communication failure, which makes it difficult to resolve the radio dead zones throughout all directions.